Sintered cobalt-rare earth bodies and method of production

ABSTRACT

By infiltrating liquid calcium metal into a green compact of single-phase Co5R powder of high-coercive force during the early stages of a sintering operation, the coercive force of the powder particles of the compact are preserved or increased and a new sintered product having high-coercive force and high-magnetic saturation is obtained.

United States Patent Cech 1 Feb. 1, 1972 [54] SINTERED COBALT-RARE EARTHStrnat et a1 ..75/200 X 3,440,043 4/1969 Zdanuk et a1. ..29/182.1 X3,505,065 4/1978 Gwyne, Jr ....29/l82.1 X 3,560,200 2/1971 Nesbitt et a1..148/31 .57 X

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. WhiteAttorneyRichard R. Brainard, Paul A. Frank, Charles T. Watts, Frank L.Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT Byinfiltrating liquid calcium metal into a green compact of single-phaseCo R powder of high-coercive force during the early stages of asintering operation, the coercive force of the powder particles of thecompact are preserved or increased and a new sintered product havinghigh-coercive force and high-magnetic saturation is obtained.

13 Claims, No Drawings SINTERED COBALT-RARE EARTH BODIES AND METHOD OFPRODUCTION The present invention relates generally to the permanentmagnet art and is more particularly concerned withnovel sinteredcobalt-rare earth intermetallic bodies having high coercive force andhigh magnetic saturation properties, and with a new method of makingthese bodies.

Cobalt-rare earth intermetallic compounds of the type C0,,R (Rdesignates a rare-earth metal or yttrium in each occurrence) possessremarkable properties of high magnetic coercive force and saturationmagnetization, and they exist in a variety of phases but exhibit theirbest magnetic properties in a single-phase form. In powder form,single-phase Co R compounds possess generally very good properties, butunfortunately they cannot be used in that form because of instability inair and resulting rapid deterioration of their magnetic characteristics.Neither has it been possible heretofore to retain the superior magneticproperties of C0,,R powders through processing into consolidated bodieswhich are not subject to such rapid deterioration. Efforts in thisdirection have included the formation of green bodies or compacts whichare sintered with or without infiltrating liquid metal to increase finaldensity.

Now, for the first time to my knowledge, it is possible through the useof the present invention to retain and even enhance the magneticproperties of Co R powders as they are consolidated into sinteredbodies. This result is enabled by my several unexpected discoveries uponwhich the novel method and the new articles of my invention arepredicated. Moreover, no substantial offsetting disadvantage is incurredin obtaining this new result.

One of these important discoveries of mine is that if calcium metal isemployed as an infiltrating material so that it melts and enters thepores and voids of the green compact during the early stages ofsintering, the superior magnetic properties of the Co R powder materialcan be retained.

Another of my important discoveries is that the high coercive force ofsuch material under certain critical circumstances can actually besignificantly increased during processing to final sintered body form.

Still another discovery that I have made is that the useful life of asintered Co R body can be greatly prolonged, the desired magneticproperties being stabilized by providing a thin metallic coating overthe entire surface of the freshly prepared sintered body.

Still another of my discoveries is that certain materials other thancalcium itself can be employed to obtain the foregoing results andadvantages, and even in some cases to make unnecessary the protectivecoating of the ultimate sintered body. Thus, as equivalents oralternatives to calcium, I have found that thorium, calcium-copper alloyand calcium rare-earth intermetallic compounds or alloys may be employedas the infiltrating protective material. Calcium hydride and thoriumhydride as sources of calcium metal and thorium metal at elevatedtemperatures or during early stages of sintering may likewise be usedwith the same ultimate advantages.

An additional discovery of mine is that while this process is applicablewith the foregoing advantages to Co Sm compositions containing from 34.8to 36.5 percent samarium when calcium, for example, is employed as theinfiltrating material, this range can be extended downward to about 31.2percent samarium if the infiltrating agent includes samarium in formsuch that it will react with the cobalt-samarium of the green compact toproduce Co Sm compounds of from 34.8 to 36.5 percent samarium.

l have additionally discovered that an amount of infiltrantsubstantially less than that required to completely fill the void volumeof the green compact Cp R body is effective to preserve or enhance themagnetic characteristics of the original C0,,R single-phase material ofthe green compact. Thus, an amount approximating one-quarter toone-third of that necessary to fill the void volume will be sufficientto provide the coating effects which I believe to be essential to theprotection of the magnetic properties through the sintering operation.

Finally, I have found that even if the void volume is substantiallyfilled with liquid calcium during sintering, a high degree ofdensification beyond that prevailing at this stage can be realized byevaporating the calcium from the sintered body under sinteringconditions which then results in the filling partially or substantiallyof the void volume with C05R singlephase material itself.

Briefly and generally described, the new method of this invention basedupon the foregoing discoveries comprises the steps of pressingcobalt-rare earth intermetallic material in single-phase powder form andthus forming a green compact, and then heating the green compact incontact with liquid metallic substance, such as calcium, under aprotective atmosphere to sinter the green compact to final density. Morespecifically, the pressing operation may involve either closed diepressing or hydrostatic pressing or a compaction operation such asextrusion, and the cobalt-rare earth material may be any cobalt-rareearth or cobalt-yttrium compound which has properties of magneticsaturation and coercive force which are desirable in the final sinteredbody product. Also, the firing temperature will be in the range fromabout 950 C. to about 1,200 O, depending upon the particular materialused as infiltrating material and the rare-earth content of Co R. Theatmosphere will be one which is nonreactive in a detrimental way withthe Co R single phase during the sintering operation; and theinfiltrating material may be other than calcium itself, such as calciumhydride, calcium-copper alloy, calcium-rare earth alloy, or it maybe ofthorium or thorium hydride.

In the preferred practice of this invention, Co Sm powder of sizeaveraging less than 44 microns, preferably 5 to 10 microns with thelargest being not over 20 microns in greatest dimension, is employed asthe magnetic material and the pressing is carried out in a closed diepressing, hydropressing or isostatic pressing operation to produce abody of the desired size or shape. Then in the next stage involving thepreliminary heating and final sintering, the resulting green compact isplaced in a heating chamber with a piece of calcium metal in contactwith the compact so that as the calcium melts, it flows by capillaryaction into the pores of the compact, more or less uniformly filling thevoid volume thereof to the extent of 30 to 40 percent of that volume,the internal surfaces defining the void volume thus being substantiallyentirely wet or coated by liquid calcium. During this stage and into thefinal stages of the sintering step, calcium reacts with oxygen in thesystem (calcium oxide being the thermodynamically stable species) withthe result that the Co Sm is effectively protected against oxygenattack.

As indicated above, it is my preference to produce sintered bodies ofmaximum density approaching that of theoretical, i.e., about percent,although satisfactory sintered magn'et products can be made inaccordance with this invention with densities as low as 70 percent. Inthe production of the lower density sintered bodies, the pressuresapplied in producing the green compacts may be of the order of 20,000psi. and the sintering operation may be terminated as soon as the (30 Rsingle phase has been sintered throughout to provide a structure havingthe requisite physical properties. The higher density bodies, which arepreferred, are produced either by prolonging the sintering operation toallow for coalescence of the particles within the body, thereby closingand substantially filling the voids after the infiltrant has beenlargely expelled, or are produced by a combination of prolongedsintering with a preliminary high-pressure green compact-forming stepinvolving the use of high pressures of the order of 100,000 p.s.i. orgreater.

In its article aspect, the present invention product in general is asintered cobalt-rare earth body of average grain size less than about 44microns and density at least 70 percent of theoretical. This bodycomprises a major proportion of singlephase Co,R intermetallic compoundand a minor proportion of a substance such as calcium distributed insegregated form through the body along the grain boundaries of thematrix material. Preferably, the minor proportion substance consists ofcalcium and calcium oxide formed as a result of reaction during thehigh-temperature processing stage of oxygen and infiltrating metal. Asalso indicated above, when calcium is used as the infiltratingsubstance, the sintered body product will preferably be provided with athin coating of the order of a few microns thickness of a protectivemetal film to prevent attack by moisture and oxygen of the air on thecalcium oxide and calcium in the grain boundaries and within the voidsof the sintered article. This coating will preferably be either ofcopper, suitably provided by an electroplating operation, or of zincwhich may be vapor-deposited. Protective coatings of other metals or ofnonmetallic materials which do not significantly impair or reduce themagnetic properties or limit the utility of these new sintered products,are also contemplated for use in the method of this invention.

The following illustrative, but not limiting, examples are offered as afurther description of the present invention and the best mode ofpracticing it;

EXAMPLE I Stoichiometric Co Sm powder of particle size less than 325mesh was pressed in an aligning magnetic field of 18 kOe (kilo-oersteds)at 120,000 p.s.i. to produce a 2.5-gram disc specimen. The specimen wasplaced in a l-mil iron foil envelope providing a limited atmosphereexchange chamber, in contact with a 0.1-gram piece of calcium. Anadditional 0.01 gram of calcium was placed in the envelope for reactionwith any oxygen which might effuse into the chamber during the sinteringstage. The envelope with its contents was then placed in a molybdenumboat and transferred to the hot zone of a helium-atmosphere furnacewhere it was subjected for 5 minutes to a temperature of 1,100" C. Afterfurnace cooling, the resulting sintered body was removed from thechamber and magnetized at 43 kilo-oersteds. lts open circuit fieldstrength was 4,500 gauss measured in a cylindrical specimen with alength to diameter ratio of 0.99 (load line slope 2.4) and its intrinsiccoercive force was 27,000 oersteds. The density of the green"die-pressed body was 70 percent but this was increased to 80.6 percentin the sintered body. These properties were again measured after thespecimen had remained in air for 2 days with the following result: Opencircuit field strength, 4,500 gauss, coercive force 27,000 oersteds. Atthat time, however, the specimen exhibited a tendency to crumble,apparently because of hydration of the calcium oxide in the grainboundaries and possibly also because of oxidation of calcium metalwithin the sintered body.

EXAMPLE II Another specimen was prepared by closed-die pressing ablended mixture of Co Sm powders analyzing 35.0 percent samarium, 65.0percent cobalt. A pressing force of 100,000 p.s.i. and an aligningmagnetic field of 18 kilo-oersteds was applied to form a green bodyhaving a pressed density of 70 percent that of fully dense material. Asmall piece of calcium metal sufficient to fill the void space wasplaced in contact with the green body inside a molybdenum foil ampul,which then was placed in an iron foil envelope. The assembly washeat-treated for 90 minutes at l,160 C. in a helium-atmosphere (at aboutatmospheric pressure) furnace with a slight temperature gradient. Thespecimen was located at the highest temperature in the gradient region.After completion of the sintering treatment, the specimen wasfumace-cooled to room temperature and its density was measured and foundto be 86 percent of theoretical. It was then copper plated, and nextheattreated for minutes at 400 C. in vacuum, and magnetized at 43kilo-oersteds for magnetic measurements. The specimen was found to havean intrinsic coercive force of 14.5 kilo-oersteds and an open circuitflux of 4,120 gauss, (load line slope 1.2). The specimen was next agedin air for 192 hours at 110C. and remeasured. The intrinsic coerciveforce was still 14.5 kilo-oersteds and the open circuit flux was 4,120gauss.

As indicated above, the rare earth alloys and intermetallic compoundsuseful in accordance with this invention are the 15 elements of theLanthanide Series having atomic numbers 57 to 71, inclusive. The elementyttrium (atomic number 39) is commonly included in this group of metalsand, for purposes of this invention and this specification, isconsidered a rareearth metal.

Representative of the cobalt-rare earth alloys and intermetalliccompounds contemplated for use in the process of this invention arecobalt-cerium, cobalt-praseodymium, cobalt-neodymium, cobalt-samarium,cobalt-europium, cobalt-gadolinium, cobalt-terbium, cobalt-dysprosium,cobalt-holmium, cobalt-erbium, cobalt-thulium, cobalt-ytterbium,cobalt-lutetium, cobalt-yttrium, cobalt-lanthanum and cobalt-mischmetal. Misch metal is presently the most common alloy of rare-earthmetals and it contains metals in the approximate ratio in which theyoccur in their most common occurring natural ores. W

As also previously indicated herein, it is contemplated by the presentinvention that cobalt-rare earth alloys or intermetallic compounds inpowder form can be processed in accordance with the present discoverieswith calcium-rare earth compounds of different rare-earth metals. Thus,for example, calcium-lanthanum of any desirable lanthanum content may beused in the processing of Co Sm powder to produce the ultimate novelsintered product of this invention. In this case, the surface portionsof the sintered body product would be ternary alloy or intermetalliccompound of cobalt-samariumlanthanum. The new advantages and results ofthe present invention may accordingly be obtained without matching thecalcium-rare earth compounds with the corresponding cobaltrare earthcompounds. It is even contemplated that mixtures of calcium-rare earthcompounds, such as calcium-lanthanum and calcium-cerium, will beemployed for the purposes of this invention where, for instance, theintermetallic compound or powder is Co Sm.

Wherever in this specification proportions or percentages are stated,reference is to the weight basis unless otherwise expressly stated. 1

Having thus described this invention in such full, clear, concise andexact terms as to enable any person skilled in the art to which itpertains to make and use the same, and having set forth the best modecontemplated of carrying out this invention, I state that the subjectmatter which I regard as being my invention is particularly pointed outand distinctly claimed in what is claimed, it being understood thatequivalents or modifications of, or substitutions for, part of thespecifically described embodiments of the invention may be made withoutdeparting from the scope of the invention as set forth in what isclaimed.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of making a high-density sintered body from cobalt-rareearth intermetallic material in powder form while retaining the highcoercive force and high magnetic saturation properties of the powderwhich comprises the steps of pressing powder of the said material whichat sintering temperature will exist as a single intermetallic phase andthereby forming a green compact, and heating the green compact incontact with a liquid metallic substance selected from the groupconsisting of calcium, thorium, calcium-copper alloy, and calcium-rareearth intermetallic compound or alloy in a protective atmosphere and ata temperature from about 950 C. to about l,200 C. and thereby sinteringthe cobalt-rare earth intermetallic green compact to final density.

2. The method of claim 1 in which the intermetallic material in powderform is Co Sm.

3. The method of claim 1 in which the liquid metallic substance iscalcium.

4. The method of claim 1 in which the liquid metallic substance iscalcium-samarium intermetallic compound or alloy.

5. The method of making a sintered cobalt-rare earth intermetallic bodyform cobalt-rare earth intermetallic material in nun-n (nan powder formwhile retaining the high coercive force and magnetic saturationcharacteristics of the powder which comprises the steps of forming agreen compact of the powder having internal surfaces defining voids,positioning a body of metallic material selected from the groupconsisting of calcium, thorium, calcium-copper alloy and calcium-rareearth intermetallic compound in contact with the green compact, heatingthe green compact and juxtaposed metallic body and melting the metallicbody and thereby infiltrating the green compact and coating the internalvoid-defining surfaces of the compact with metal from said metallicbody, and sintering the green compact to final density.

6. The method of claim 5 in which the sintering step is carried out at atemperature and for a time sufficient to evaporate substantially all themetallic coating on the internal voiddefining surfaces of the compact.

7. The method of claim 5 in which the cobalt-rare earth intermetallicmaterial is Co -,Sm and in which the body of metallic material consistsof calcium.

8. The method of claim 5 in which the step of forming the green compactis carried out by pressing the powder in a magnetic field tomagnetically align the powder particles.

9. The method of claim 5 in which the body of infiltrating metallicmaterial consists of calcium and in which the calcium is employed inamount at least sufficient to fill one-third of the total void volume ofthe green compact.

10. The method of claim 5 in which the sintering step is carried outuntil the void volume is substantially filled with cobalt-rare earthintermetallic material.

11. The method of claim 5 in which calcium is the infiltrating metallicmaterial and in which the resulting sintered intermetallic body isprovided with a protective coating of copper or zinc as the finalprocessing step.

12. A sintered cobalt-rare earth body having an average grain size lessthan about 44 microns, having a density of at least about percent oftheoretical and consisting essentially of a major proportion of Co Rintermetallic phase and a minor proportion of a substance selected fromthe group consisting of calcium, thorium, calcium oxide, thorium oxide,calciumcopper alloy and calcium-rare earth intermetallic compound oralloy, and mixtures thereof, said minor proportion substance beingsegregated in the body along the grain boundaries of the Co Rintermetallic phase.

13. The sintered body of claim 12 in which the major proportioninterrnetallic phase consists of Co Sm and the minor proportionsubstance consists of calcium and calcium oxide.

2. The method of claim 1 in which the intermetallic material in powderform is Co5Sm.
 3. The method of claim 1 in which the liquid metallicsubstance is calcium.
 4. The method of claim 1 in which the liquidmetallic substance is calcium-samarium intermetallic compound or alloy.5. The method of making a sintered cobalt-rare earth intermetallic bodyform cobalt-rare earth intermetallic material in powder form whileretaining the high coercive force and magnetic saturationcharacteristics of the powder which comprises the steps of forming agreen compact of the powder having internal surfaces defining voids,positioning a body of metallic material selected from the groupconsisting of calcium, thorium, calcium-copper alloy and calcium-rareearth intermetallic compound in contact with the green compact, heatingthe green compact and juxtaposed metallic body and melting the metallicbody and thereby infiltrating the green compact and coating the internalvoid-defining surfaces of the compact with metal from said metallicbody, and sintering the green compact to final density.
 6. The method ofclaim 5 in which the sintering step is carried out at a temperature andfor a time sufficient to evaporate substantially all the metalliccoating on the internal void-defining surfaces of the compact.
 7. Themethod of claim 5 in which the cobalt-rare earth intermetallic materialis Co5Sm and in which the body of metallic material consists of calcium.8. The method of claim 5 in which the step of forming the green compactis carried out by pressing the powder in a magnetic field tomagnetically align the powder particles.
 9. The method of claim 5 inwhich the body of infiltrating metallic material consists of calcium andin which the calcium is employed in amount at least sufficient to fillone-third of the total void volume of the green compact.
 10. The methodof claim 5 in which the sintering step is carried out until the voidvolume is substantially filled with cobalt-rare earth intermetallicmaterial.
 11. The method of claim 5 in which calcium is the infiltratingmetallic material and in which the resulting sintered intermetallic bodyis provided with a protective coating of copper or zinc as the finalprocessing step.
 12. A sintered cobalt-rare earth body having an averagegrain size less than about 44 microns, having a density of at leastabout 70 percent of theoretical and consisting essentially of a majorproportion of Co5R intermetallic phase and a minor proportion of asubstance selected from the group consisting of calcium, thorium,calcium oxide, thorium oxide, calcium-copper alloy and calcium-rareearth intermetallic compound or alloy, and mixtures thereof, said minorproportion substance being segregated in the body along the grainboundaries of the Co5R intermetallic phase.
 13. The sintered body ofclaim 12 in which the major proportion intermetallic phase consists ofCo5Sm and the minor proportion substance consists of calcium and calciumoxide.